1. Field of the Invention
The present invention relates generally to contamination inspection for semiconductor wafers and the like and in particular to a system which inspects both the frontside and backside of a semiconductor wafer without manual or automatic inversion of the wafer.
2. Description of the Related Art
Tools used in the semiconductor wafer manufacturing process must periodically be checked to determine whether they must be replaced or are still in usable condition. The condition of a tool is checked by inspecting wafers processed by that tool for defects. Bare wafers are typically routed through the process tool with the frontside facing up, and wafer defects detected optically by illuminating portions of the wafer and measuring the amount of illuminating light scattered by defects on the wafer surface.
Previously, systems which performed inspection of wafers did so in two discrete stages. First, the frontside of the wafer was scanned for contamination caused by the process tool. If the defect rate on the frontside of the wafer was acceptable, the wafer was then turned over to inspect the backside for further particle contamination and other defects. The process tool was considered usable if the defect rate on the backside of the wafer was also acceptable.
Inspection of both sides of a wafer by these procedures accordingly required time for inspection of one side, examination of the one side, inverting the wafer without excessively damaging the wafer, scanning the reverse side, and examining the results of the second side scan. In addition to this excessive amount of time required for examination, the process of flipping the semiconductor wafer had a tendency to contaminate the edges of the wafer due to surface or edge contact with a gripping device. In some processes, when the wafer was flipped over to inspect the backside, the front side of the wafer could be contaminated by the flipping process. The resulting contamination of the frontside of the wafer tends to render the wafer unsuitable for further processing. Thus, all test wafers were usually scrapped after each inspection, reducing overall productivity and increasing per unit cost.
Edge handling of wafers has also complicated the problem. As wafers tend to suffer from contamination or other degradation when handled by wafer orientation systems, the handling of a wafer requires special care. Although previous wafer orientation systems have included multiple drive rollers, radially inwardly-biased contact rollers, and a tiltable wafer-supporting table with an air-bearing mechanism, each of these handling methods have benefits and drawbacks. Systems without multiple drive rollers and radially inwardly-biased or spring-loaded contact rollers cannot maintain steady wafer rotation rate during the portion of a cycle in which the drive roller is not in contact with the round edge of the wafer because the drive roller loses traction along the wafer edge.
In inspection equipment, it is important to maintain steady rates of wafer rotation to avoid errors in defect detection, such as errors in detecting defects where none exist, or simply failing to detect defects. Previous systems which supported semiconductor wafers through direct contact with a solid surface present special problems during inspection since contact with the support surface may increase contamination or move defects from one location to another in ways that render the wafer unsuitable for future processing.
It is therefore an object of the current invention to provide a system for minimizing the time required for full inspection of both the front side and back side of a wafer.
It is another object of the current invention to provide an arrangement which minimizes overall wafer contamination during the inspection process, particularly when inspecting both front and back sides of the wafer.
It is a further object of the current invention to minimize edge handling concerns, such as contamination, during the inspection of the front side and back side of a wafer.
It is still a further object of the current invention to minimize the number of defects missed or falsely detected by the inspection system.